Organic Light Emitting Diode (OLED) displays were a $1.25 billion market in 2010, which is projected to grow annually at a rate of 25%. The high efficiency and high contrast ratio of OLED displays make them a suitable replacement for liquid crystal displays (LCDs) in the mobile phone display, digital camera, and global positioning system (GPS) market segments. These applications place a premium on high electrical efficiency, compact size, and robustness. This has increased the demand for active matrix OLEDs (AMOLEDs) which consume less power, have faster response times, and higher resolutions. AMOLED innovations that improve these properties will further accelerate AMOLED adoption into portable devices and expand the range of devices that use them. These performance factors are largely driven by the processing temperature of the electronics. AMOLEDs have a thin-film transistor (TFT) array structure which is deposited on the transparent substrate. Higher TFT deposition temperatures can dramatically improve the electrical efficiency of the display. Currently, glass plates are used as AMOLED substrates. They offer high processing temperatures (>500° C.) and good barrier properties, but are relatively thick, heavy, rigid, and are vulnerable to breaking, which reduces product design freedom and display robustness. Thus, there is a demand by portable device manufacturers for a lighter, thinner, and more robust replacement. Flexible substrate materials would also open new possibilities for product design, and enable lower cost roll-to-roll fabrication. Such substrates could also be used in the production of liquid crystal displays.
Many polymer thin films have excellent flexibility, transparency, are relatively inexpensive, and are lightweight. Polymer films are excellent candidates for substrates for flexible electronic devices, including flexible displays and flexible solar cell panels, which are currently under development. Compared to rigid substrates like glass, flexible substrates offer some potentially significant advantages in electronic devices, including:
a. Light weight (glass substrates represent about 98% of the total weight in a thin film solar cell).
b. Flexible (Easy to handle, low transportation costs, and/or more applications for both raw materials and products.)
c. Amenable to roll-to-roll manufacturing, which could greatly reduce the manufacturing costs.
To facilitate these inherent advantages of a polymeric substrate for the flexible display application, several issues must be addressed including:
a. Increasing the thermal stability;
b. Reducing the coefficient of thermal expansion (CTE);
c. Maintaining high transparency during high temperature processing; and,
d. Increasing the oxygen and moisture barrier properties. Currently, no pure polymer film can provide sufficient barrier properties. To achieve the target barrier property, an additional barrier layer must be applied.
Several polymer films have been evaluated as transparent flexible substrates, including: polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyethersulfone (PES), cyclic olefin polymer (COP), polyarylates (PAR), polyimides (PI), and others. However, no one film can meet all the requirements. Currently, the industrial standard for this application is PEN film, which meets part of the requirements (Transmittance>80% between 400 nm and 750 nm), but has a limited use temperature (<200° C.). A transparent polymer film with a higher thermal stability (Tg>300° C.) is needed to allow a flexible electronic device to be fabricated at the desired higher temperatures.
Conventional aromatic polyimides are well known for their excellent thermal and mechanical properties, but their films, which must be cast from their polyamic acid precursors, are usually dark yellow to orange. Some aromatic polyimides have been prepared that can be solution cast into films that are colorless in the visible region, but such films still absorb light near 400 nm and do not display the required low CTE (For example, F. Li. F. W. Harris, and S. Z. D. Cheng, Polymer, 37, 23, pp 5321 1996). The films are also not solvent resistant. Polyimide films based on part or all alicyclic monomers, such as those described in patents JP 2007-063417 and JP 2007-231224, and publication by A. S. Mathews et al (J. Appl. Polym. Sci., Vol. 102, 3316-3326, 2006), show improved transparency. Although Tgs of these polymers can be higher than 300° C., at these temperatures, the polymers do not show sufficient thermal stability due to their aliphatic units.
Fiber reinforced polymer composite films, such as reported by H. Ito (Jap. J. Appl. Phys., 45, No. 5B, pp 4325, 2006), combine the dimensional stability of fiber glass in a polymer film, offering an alternative way to achieve a low CTE. However, in order to maintain a high transparency, the refractive indices of the matrix polymer and the fiber must be precisely matched, which greatly limits the choice of the matrix polymer within an organic silicon resin. By using nanoparticles as filler, the effect of lowering CTE is not significant (J M Liu, et al, J. SID, Vol. 19, No. 1, 2011)
Although most aromatic polyamides are poorly soluble and cannot be solution cast into films, a few polymers have been prepared that are soluble in polar aprotic solvents containing inorganic salts. Some of these have been investigated for use as flexible substrates. For example, JP 2009-79210A describes a thin film prepared from a fluorine containing aromatic polyamide that displays a very low CTE (<0 ppm/° C.), good transparency (T %>80 between 450˜700 nm), and excellent mechanical properties. However, the maximum thickness of films made from this polymer is 20 μm, because a dry-wet method where the residual salt is removed must be used for the film preparation. The film also displays poor resistance to strong organic solvents, which greatly reduces its utility. In fact, there have been few attempts to increase the solvent resistance of aromatic polyamides through crosslinking because most are inherently solvent resistant. However, one method to increase the solvent resistance of soluble aromatic polyamides using multifunctional epoxides has been described in PCT WO2013/006452 A2.